RISC-V: expose extension bits in AT_HWCAP

AT_HWCAP is a field in the elf auxiliary vector meant to describe
cpu-specific hardware features. For RISC-V we want to use this to
indicate the presence of any standard extensions supported by the CPU.
This allows userland applications to query the system for supported
extensions using elf_aux_info(3).

Support for an extension is indicated by the presence of its
corresponding bit in AT_HWCAP -- e.g. systems supporting the 'c'
extension (compressed instructions) will have the second bit set.

Extensions advertised through AT_HWCAP are only those that are supported
by all harts in the system.

Reviewed by: jhb, markj
Approved by: markj (mentor)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D20493